Why is this not Rosa canina?

Are you saying that it is Rosa canina? if so, what are you seeing that tells you that it's canina rather than something else?

Photos I've seen of Rosa canina do not resemble this rose. The thorns are longer and more hooked and all in pairs; the sepals don't have any lateral lobes; and the flowers are all pink, none white.

Flora of the Pacific Northwest leads one to assume that any nonnative rose that isn't eglanteria must be canina, but there are many naturalized roses out there that aren't either one. That's why I'm looking for a botanical key to species roses.

Dog rose, a variable species, includes plants with white flowers. You can find keys on the internet not specific to our region, for instance the one in the online Jepson manual - where both sweetbriar and dog rose appear near one another. But the rest of that key consists only of species native to California.

The Rosa portion of Flora of North America is still being worked on. Older printed sources with keys include Bailey's MANUAL OF CULTIVATED PLANTS and Rehder's MANUAL OF HARDY TREES AND SHRUBS. Again, not limited to roses seen wild in this area.

List (with remarks) of roses seen wild in Seattle, including garden forms is in back of Jacobson's WILD PLANTS OF GREATER SEATTLE. Web site of Burke Herbarium (at University of Washington) shows photos, distribution by county and mentions morphological features of roses represented by specimens held there. Dog rose and sweetbriar are both among them.

Your upper picture has leaves like sweetbriar, bottom plant looks quite typical of dog rose.